Support sancov

[kennytm]

• [ ] (rustc) Make sure asan is supported on all relevant platforms (tracking)
• [ ] (cargo-cov) Build sancov-enabled binaries (easy)
• [ ] (cargo-cov) Run sancov-enabled binaries to get the *.sancov files (easy)
• [ ] (cov) Construct Graph by analyzing the DWARF info (very hard)

https://clang.llvm.org/docs/SanitizerCoverage.html